The thermosensitive recording material expresses the image area and the non-image area as a temperature difference distribution and many systems therefor have been proposed, such as transfer by the melting or sublimation of a coloring agent, color formation reaction between two components due to heat-melting or rupture of a capsule, or change in the optical properties due to phase transition. These kinds of thermo-sensitive recording mediums are advantageous in that a recording image can be obtained by a dry and simple system and the maintenance is not necessary, thus, are widely used as an output material of various printers, word processors, facsimiles and the like. Furthermore, to keep up with the progress of laser recording devices in recent years, application of the thermosensitive recording medium to optical disks or photomechanical materials is being studied.
The photomechanical material heretofore used is a silver halide light-sensitive material requiring a wet processing. However, because of demands for simplification of the processing steps and problems of environmental pollution by the processing solutions, development of a dry process is required and several technical proposals using a thermosensitive recording system have been recently made. In view of the resolution, recording of an image using a laser is preferred and for example, a system using a high output laser, called dye ablation, has been developed. Recording materials therefor are disclosed in JP-A-7-164755 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-7-149063 and JP-A-7-149065, and image forming apparatuses are disclosed in JP-A-8-48053 and JP-A-8-72400. According to this system, a recording material comprising a support having coated thereon a dye composition comprising an image dye, a substance having an absorption in the laser wavelength region (infrared ray absorptive substance) and a binder is irradiated with a laser beam from the dye layer side to record an image. The energy given by the laser beam brings about abrupt local changes in the image forming layer at the spot of the laser beam thrusting the material and thereby expels the substance from the layer. According to the above-described patent publications, this is not a completely physical change (for example, melting, evaporation or sublimation) but a kind of chemical change (for example, bond destruction), and the image dye is not partially removed but completely removed. The dye ablation system has, however, problems that a high output laser is indispensable so as to increase the dye removal efficiency at the site exposed to a laser beam and a dust collector for collecting dyes removed is necessary to be installed in combination.
As a system which can dispense with a dust collector, an ablation transfer image recording method using a laser heat source is disclosed in U.S. Pat. No. 5,171,650. This system uses a dye donor sheet containing a dynamic release layer formed by overcoating an ablative carrier topcoat, where an image is transferred to a separate receiving sheet which is placed adjacently and aligned. Therefore, the sheet left useless after the recording of an image raises a problem of waste materials. Furthermore, also in this case, a high output laser is indispensable so as to increase the transfer efficiency. Thus, conventional thermosensitive recording systems using ablation by a laser cannot dispense with a high output laser and are bound to the problem of dusts or waste materials.
On the other hand, a method expanded from a system called "dry silver" is described in JP-A-6-194781 as a heat-recording system involving no ablation using a laser. According to this system, recording is performed by a laser on a recording material containing a silver source capable of thermal reduction, a reducing agent for silver ion and a light-heat conversion dye. This system is, however, deficient in the practical performance in view of storability of the non-image area and heat sensitivity.
Another example of the thermosensitive recording system using a laser is a compound capable of changing the absorption due to thermal decomposition of carbamate described in U.S. Pat. Nos. 4,602,263 and 4,826,976. U.S. Pat. No. 5,243,052 describes a compound of forming a yellow color due to thermal decomposition of the t-butoxycarbonyl group introduced into the hydroxyl group. These systems use an irreversible monomolecular reaction and are suited for the image recording within a short time using a laser, however, the sensitivity is not sufficiently high and improvement for higher sensitivity is needed. Higher sensitivity may be achieved by allowing an acid catalyst present together but in turn a problem arises in the storage stability.
With respect to the method for forming a UV mask image (360 nm to 420 nm; corresponding to the exposure light source for PS plate) used for the photomechanical material, heat mode systems using a laser have not been proposed in practice.
U.S. Pat. Nos. 5,286,612, 5,395,736 and 5,441,850, WO94/10606 and WO94/10607 describe a recording material comprising a compound which generates an extra strong acid of pKa&lt;0 by the action of an ultraviolet ray, a secondary acid generating agent which generates a secondary acid in the presence of an extra strong acid, and a compound which changes in the color upon contacting with the secondary acid. This system is effective for achieving high sensitivity of a recording material. However, generation of the secondary acid from a secondary acid generating agent described in those patent publications is catalyzed only by an extra strong acid generated from an extra strong acid precursor and although the patent publications state that about 20 molecules of the secondary acid precursor are catalyzed to decompose per one molecule of the extra strong acid, the secondary acid produced has no catalytic capability. Accordingly, increase in the sensitivity by the acid breeding in geometrical progression cannot be expected.
As a compound which breeds an acid by the action of an acid, K. Ichimura, Chem. Lett., 551 (1995) and JP-A-8-248561 describe a compound capable of producing an organic acid due to decomposition by an acid catalyst. In these publications, an acid reactive polymer composition comprising a combination of a photoacid generating agent and a substance of causing structural change by the action of an acid is described. However, the structural change caused by the action of an acid generated from a photoacid generating agent is to cause solubilization or insolubilization in a polar solvent or an aqueous alkali solution and the end use thereof is a so-called photoresist. These publications are completely silent on the recording material using change in the absorption.
The present inventors have made extensive investigations on the compounds developed by Ichimura et al and have found that the compounds reported by Ichimura et al mostly have a function as a thermal acid generating agent and can be used for the image formation because of their capability to act as an acid generating agent even if a photo or thermal acid generating agent is not newly added.